Preheat the oven to 425°F (220°C).
Put the potatoes in a large microwave-safe bowl and fill the bottom with 1/2 inch (12 mm) of water. Cover with plastic wrap and poke the wrap a few times with a fork. Microwave on high until the potatoes can be easily pierced with a fork, about 10 minutes, stirring once, and replenishing the water as needed. Remove, uncover, drain well, and cool until just warm.
Using the heel of your hand, smash the potatoes into rounds. (You want them to hold together, so be gentle. Love your potatoes, even when you’re smashing them in the face.)
Pour the oil onto a large rimmed baking sheet and sprinkle the oil with 1/2 teaspoon of the salt and 1/2 teaspoon of the pepper. Arrange the smashed potatoes on the sheet and sprinkle the tops with the remaining 1/2 teaspoon salt and 1/2 teaspoon pepper. Put it on the floor of the oven and roast until the undersides are crispy but not burnt, 10 to 20 minutes.
Remove the baking sheet from the oven and use tongs or a spatula to flip the potatoes, then return to the oven and roast until the undersides are crispy, 10 to 20 minutes more.